1. Technical Field
The invention relates to a transmitting device limiting out-of-band interferences.
Digital modulation is used more and more for satellite and terrestrial transmission. Several types of modulation are used, but they all consist in the transmission of symbols which are coded in terms of phase and of amplitude, possibly on several carriers.
2. Prior Art
In theory, it is possible to carry out direct modulation, that is to say to modulate the carrier frequency directly. However, the digital systems are generally placed in internal units which are not subjected to strong variations in temperature, whereas the antennae are placed outside at some distance. For this reason, an intermediate-frequency band, lower than the transmitting-frequency band, is generally used to form the link between the internal unit and the external unit, frequency transposition being carried out in the external unit.
Moreover, if a multi-channel type transmission system is used, it is necessary to have recourse to variable channel transposition when it is not possible to have frequency agility upon modulation. A frequency-transposition stage employing an oscillator makes it possible to provide the frequency agility.
Given the nature of the modulation, it proves simpler to carry out the modulation in digital mode. However, integrated circuits have operating limits, especially as regards digital/analog converters. In order to transmit a digital signal, it is known to have recourse to low-frequency modulation followed by a transposition to a higher frequency at which the conjugate image of the signal to be transmitted is eliminated more easily.
It is known, for example, to have recourse to a transmission path as represented in FIG. 1. The transmission path of FIG. 1 includes a first stage 10 carrying out modulation at “low” frequency, a second stage 20 serving for transposing the useful signal and filtering the image band of the signal to be transmitted, a third stage 30 producing the frequency agility, and a fourth stage 40 carrying out the transposition of the signal to the transmitting frequency.
As is known in prior art, a coding device, not represented, codes the data and supplies modulating signals R(s) and I(s) which correspond, for example, to a train of complex symbols to be transmitted. The modulation takes place, for example, according to a known technique by the use of two mixers 11 and 12 each receiving a modulating signal R(s) and I(s) and a signal of given frequency Cos(ωt) or Sin(ωt). An adder circuit 13 adds the signals originating from the mixers 11 and 12. A filter 14 keeps the useful part of the signal originating from the adder circuit 13. FIG. 2A shows the spectrum of the signal leaving the first stage 10. The first stage 10 is generally implemented by the use of a circuit which carries out the mixing, the adding and the filtering digitally. The signal is converted into analog at the output of the first stage 10 by the use of a converter, not represented. For reasons of scaling and of feasibility of the integrated circuit, modulation takes place at “low” frequency, that is to say that the modulated signal is, for example, below 50 MHz. Under such modulation conditions, it may happen that the useful band of the signal (which corresponds, for example, to a transmission channel) occupies a wide spectrum, for example 30 MHz, at low frequency, for example between 20 and 50 MHz.
The second stage 20 carries out frequency transposition and filtering which are for the purpose of shifting the useful band of the signal into higher frequencies. FIG. 2B represents the transposed useful band 100, the conjugate and transposed image 101 of the useful band, a spectral line 102 at the frequency of the local oscillator LO1 corresponding to a leakage from the mixer 21, and the template 103 of the filter 22 of the second stage 20. By way of example, if the frequency of the local oscillator LO1 is at 300 MHz, the useful band lies centred on 335 MHz between 320 and 350 MHz. The filter 22 has to reject the conjugate image 101 lying between 250 and 280 MHz and eliminate the spectral line 102 situated at 300 MHz. Such constraints on the filter 22 are very severe and difficult to achieve. Before filtering, the conjugate image 101 is substantially of the same power as the useful band 100 and the spectral line 102 generally remains present.
The third stage 30 implements the frequency agility by the use of a mixer 31, of a local oscillator LO2 of variable frequency and of a filter 32 which suppresses the image and harmonic frequencies of the transposition. FIG. 2C represents the extreme transpositions 104 of the useful band, the template 105 of the filter 32 and the parasitic transpositions 106 and 107 of the parasitic components 101 and 102. By way of example, the variable oscillator LO2 supplies a signal with frequency lying between 1.4 and 1.9 GHz so as to transpose the useful band into a range lying between 1.72 and 2.25 GHz. The template 105 is for the purpose of suppressing the image and harmonic frequencies situated outside this range. The parasitic transpositions 106 included in the range cannot be attenuated and the transmission system has to take them into account in order to minimize the impact on the quality of the transmission. The parasitic transpositions 107 lying outside the range are slightly attenuated since the filter 32 is a wideband filter which should not degrade the useful signal and since these parasitic transpositions 107 are very close to the useful signal.
The fourth stage 40 is generally situated close to the antenna and carries out the transposition into the transmission band, lying, for example, between 14 and 14.53 GHz, as well as the power amplification so as to transmit the signal. FIG. 2D represents the spectrum of the transmitted signal, the transmission band 108 and out-of-band interferences 109 transmitted when the useful band is transmitted at the limit of the transmission band.
The transmission bands are allocated by regulatory authorities. The regulatory authorities set the conditions for use of these bands, and especially the maximum noise level radiated outside the allocated band, in a very strict way, so as not to upset the users of adjacent bands. The parasitic effects 109 situated outside the band have to be below a threshold determined by the monitoring authorities. This threshold may be very low. The attenuation of the out-of-band interferences with respect to the useful signal is 60 dB, for example.
The person skilled in the art is confronted with several solutions for complying with the variable thresholds. A first solution consists in not using all of the allocated band, such that the part 111 lies in the allocated frequency band. A second solution consists in using high-rejection filters, which are very difficult to produce and which may distort the signal and degrade the performance of the transmission system. A third solution consists in carrying out the modulation at higher frequency, which amounts to spacing the image spectrum and the spectral line of the local oscillator away from the useful signal, but which dictates carrying out the modulation in analog mode, with the use of lower-performance discrete components.